If you hold a cat by the tail you learn things you cannot learn any other way.” - Mark Twain-

PERIODIC TABLE TRENDS

TO UNDERSTAND THE PROPERTIES OF ATOMS OF AN ELEMENT, WE NEED TO KNOW THE ELECTRON CONFIGURATION OF THE ATOMS, ESPECIALLY HOW MANY VALENCE (OUTER) ELECTRONS THE ATOM HAVE.

WE ALSO NEED TO UNDERSTAND HOW STRONGLY THE OUTER ELECTRONS ARE ATTRACTED BY THE PROTONS IN THE NUCLEUS.

THE ELECTROSTATIC FORCE OF ATTRACTION BETWEEN CHARGED OBJECTS IS GIVEN BY COULOMB’S LAW AS

F = kQ1Q2/d2

WHERE F = FORCE, k = CONSTANT, Q1 and Q2 are charges, and d = distance of separation

SO, THE FORCE OF ATTRACTION BETWEEN THE NUCLEUS AND AN OUTER ELECTRON DEPENDS ON THE CHARGE ON THE NUCLEUS AND THE DISTANCE BETWEEN THE ELECTRON AND THE NUCLEUS.

THE FORCE OF ATTRACTION INCREASES AS THE NUCLEAR CHARGE INCREASES AND DECREASES AS THE ELECTRON MOVES FURTHER FROM THE NUCLEUS.

NOW, IT WOULD BE DIFFICULT TO CALCULATE THE ACTUAL FORCE AN OUTER ELECTRON EXPERIENCES IN AN ATOM WITH A NUMBER OF ELECTRONS.

BUT, WE CAN GET AN IDEA BY THINKING IN TERMS OF EFFECTIVE NUCLEAR CHARGE.

THINK OF THE INNER ELECTRONS AS SHIELDING THE OUTER ELECTRONS FROM THE NUCLEUS.

ANOTHER PICTURE THAT ILLUSTRATES THIS EFFECT:

SO, WE COULD THINK OF THE CHARGE AN OUTER ELECTRON MIGHT BE SEEING AS:

Zeff = Z – S WHERE Z = ATOMC NUMBERS = # OF INNER

ELECTRONS

LET’S TAKE A COUPLE OF EXAMPLES:

SODIUM HAS AN ATOMIC NUMBER OF 11 AND ITS ELECTRON CONFIGURATION IS

1s22s22p63s1

Inner electrons

SO, Zeff = 11-10 = 1

IF WE TAKE NEON, Ne, WHICH HAS Z = 10, THE ELECTRON CONFIGURATION IS:

1s22s22p6

Inner electrons

SO, THE EFFECTIVE NUCLEAR CHARGE AN OUTER ELECTRON WOULD EXPERIENCE IS:

Zeff = 10 – 2 = 8

THIS SHIELDING EFFECT BY INNER ELECTRONS WOULD HELP EXPLAIN HOW ATOMIC SIZE VARIES IN THE PERIODIC TABLE.

THERE ARE TWO TRENDS IN ATOMIC SIZE:

1. WITHIN A GROUP (FAMILY) THE SIZE TENDS TO INCREASE AS WE MOVE FROM TOP TO BOTTOM. AS WE MOVE DOWN IN A GROUP, THE OUTER ELECTRONS ARE IN SHELLS WITH HIGHER PRINCIPAL QUANTUM NUMBERS, n. THE OUTER ELECTRONS SPEND MORE TIME FURTHER FROM THE NUCLEUS.

2. WITHIN A GIVEN ROW, THE ATOMIC SIZE TENDS TO DECREASE AS WE MOVE FROM LEFT TO RIGHT. THE EFFECTIVE NUCLEAR CHARGE, Zeff , IS INCREASING, AND THE ATTRACTION FOR THE ELECTRONS BY THE NUCLEUS IS INCREASING.

COMMENTS ON ION SIZE:

1.CATIONS (POSITIVE IONS) ARE SMALLER THAN THEIR PARENT ATOMS. OUTER ELECTRONS HAVE BEEN LOST, AND THE NUCLEAR CHARGE REMAINS THE SAME. THERE IS ALSO LESS ELECTRON-ELECTRON REPULSION.2.ANIONS ARE LARGER THAN THEIR PARENT ATOMS. THE NUCLEAR CHARGE REMAINS THE SAME, AND THERE IS MORE ELECTRON-ELECTRON REPULSION.3.AS YOU MOVE DOWN IN A GROUP OR FAMILY, ION SIZE INCREASES.

IONIZATION ENERGY

THE IONIZATION ENERGY IS THE ENERGY REQUIRED TO REMOVE AN ELECTRON FROM A GASEOUS ATOM.

Na (g) Na+ (g) + e-

REMOVAL OF SUCCESSIVE ELECTRONS WOULD REQUIRE MUCH GREATER ENERGIES.

LET’S TAKE A LOOK AT HOW FIRST IONIZATION ENERGIES (ENERGY TO REMOVE THE FIRST ELECTRON) VARY IN THE PERIODIC TABLE.

THE FACTORS THAT INFLUENCED ATOMIC SIZE ALSO INFLUENCE IONIZATION ENERGIES. THE ENERGY NEEDED TO REMOVE AN ELECTRON FROM THE OUTER SHELL DEPENDS BOTH ON THE EFFECTIVE NUCLEAR CHARGE AND THE DISTANCE OF THE ELECTRON FROM THE NUCLEUS.

As proton # increases. More protons means greater attraction between nucleus and outer electron thus higher ionization energy.The greater attraction also means that outer electrons are brought closer to the nucleus, thus smaller atomic radius results.

Li (enc = 1)

+++

Be (enc = 2)

++++

B (enc = 3)

+++++

Ionization energy vs. atomic numberIonization energy vs. atomic number

Ca

K

H

He

Li

B

Be C

N O

F

Ne

Na

MgAl

SiP S

Cl

Ar

0

500

1000

1500

2000

2500

0 2 4 6 8 10 12 14 16 18 20

Element

Ioni

zatio

n en

ergy

(kJ

/mol

)

Atomic radius vs. atomic numberAtomic radius vs. atomic number

Ca

K

H He

Li

B

Be

C N O FNe

NaMg

Al Si P S Cl

Ar

0

50

100

150

200

250

0 2 4 6 8 10 12 14 16 18 20

Element

Ato

mic

Rad

ius

(pm

)

ATOMS WITH LOW IONIZATION ENERGIES WILL TEND TO FORM POSITIVE IONS. THOSE WITH HIGH IONIZATION ENERGIES WILL NOT.

THE ELECTRON AFFINITY IS THE ENERGY GAIN WHEN A NEUTRAL GASEOUS ATOM GAINS AN ELECTRON.

Cl (g) + e- Cl- (g) E = -349 kJ/mole

Note: the negative sign indicates that energy is released. The greater the attraction between an atom and an electron, the more negative the electron affinity.

THE ELECTRON AFFINITIES FOR THE NOBLE GASES ARE GREATER THAN ZERO, AS IT WOULD REQUIRE PUTTING AN ELECTRON IN A HIGHER SHELL.

THE ELECTRON AFFINITIES FOR GROUP 5A ELEMENTS ARE UNUSUAL. THESE ATOMS HAVE HALF-FILLED P SUBSHELLS, SO AN ADDITIONAL ELECTRON WOULD HAVE TO GO INTO A ORBITAL THAT IS ALREADY OCCUPIED WITH ONE ELECTRON.

NOW, WHAT DO IONIZATION POTENTIALS AND ELECTRON AFFINITIES TELL US ABOUT THE ELEMENTS.

ELEMENTS WHOSE ATOMS HAVE LOW IONIZATION POTENTIALS AND LOW ELECTRON AFFINITIES WILL TEND TO LOSE ELECTRONS AND FORM POSITIVE IONS.

THESE ARE METALS.

ELEMENTS WHOSE ATOMS HAVE HIGH IONIZATION POTENTIALS AND HIGH ELECTRON AFFINITIES WILL TEND TO GAIN ELECTRONS AND FORM NEGATIVE IONS.

THESE ARE NONMETALS.

METALS

1.METALS TEND TO HAVE LOW IONIZATION ENERGIES AND TEND TO FORM POSITIVE IONS2.HAVE A SHINY LUSTER AND MOST ARE SILVERY3.SOLIDS ARE MALLEABLE AND DUCTILE4.GOOD CONDUCTORS OF HEAT AND ELECTRICITY5.TEND TO FORM CATIONS IN AQUEOUS SOLUTION6.MOST METAL OXIDES ARE IONIC SOLIDS THAT ARE BASIC

NONMETALS

1.NONMETALS HAVE HIGH ELECTRON AFFINITIES AND TEND TO GAIN ELECTRONS TO FORM NEGATIVE IONS.2.DO NOT HAVE A LUSTER.3.SOLIDS ARE USUALLY BRITTLE.4.POOR CONDUCTORS OF HEAT AND ELECTRICITY.5.MOST NONMETAL OXIDES ARE MOLECULAR SUBSTANCES THAT FORM ACIDIC SOLUTIONS.

METALLOIDS HAVE PROPERTIES BETWEEN THOSE OF METALS AND NONMETALS. A GOOD EXAMPLE IS SILICON. IT LOOKS LIKE A METAL, BUT IS VERY BRITTLE AND IS A NONCONDUCTOR OF ELECTRICITY.

Definition Trend

ATOMIC RADIUS

RADIUS OF ATOM INCREASES AS YOU MOVE DOWN IN A GROUP AND FROM RIGHT TO LEFT ON PERIODIC TABLE

IONIZATION ENERGY

THE ENERGY REQUIRED TO REMOVE THE OUTERMOST ELECTRON FROM AN ATOM

INCREASES AS YOU MOVE UP AND TO THE RIGHT

ELECTRON AFFINITY

ENERGY RELEASED WHEN AN ELECTRON IS ADDED TO THE OUTERMOST SHELL OF AN ATOM

INCREASES IN MAGNITUDE AS YOU MOVE UP AND TO THE RIGHT

IONIC RADIUS

RADIUS OF ION •INCREASES DOWN IN A GROUP•CATIONS DECREASE LEFT TO RIGHT•ANIONS DECREASE LEFT TO RIGHT